Natural gas can be used in place of diesel for supplying a Diesel-cycle engine with fuel to obtain emission and economic benefits. In these engines it is known to use diesel as a pilot fuel since the auto-ignition temperature of natural gas is substantially greater than that of diesel. A small amount of diesel, normally about 5% of total fuel introduced to the combustion chamber, is injected along with natural gas, which is the main fuel. The diesel ignites due to compression heat and subsequently the natural gas is ignited due to the combustion of diesel.
A dual fuel injector separately injects two fuels into a combustion chamber of an internal combustion engine. In particular, the two fuels can be controlled quantities of a liquid pilot fuel, such as diesel, and a gaseous fuel such as natural gas. U.S. Pat. No. 6,336,598 (the '598 patent), issued Jan. 8, 2002, which is co-owned along with the present application by the Applicant, discloses such a dual fuel injector that is hydraulically actuated. The fuel injector comprises an injector body, and hydraulic fluid, liquid fuel and gaseous fuel inlet ports. The hydraulic fluid inlet port enables pressurized hydraulic fluid to be introduced into the interior of the injector body. A liquid seal in the injector inhibits or prevents leakage of high-pressure gaseous fuel into the hydraulic actuating fluid. The liquid seal is filled with the pressurized hydraulic fluid, which is substantially confined therein. The hydraulic fluid is of sufficient pressure to maintain sealing and to inhibit or prevent leakage of the gaseous fuel into the hydraulic fluid. In a preferred embodiment, the liquid pilot fuel and hydraulic fluid are the same and both are supplied to the dual fuel injector from the diesel common rail. When the pilot fuel is used for sealing, the gaseous fuel is pressurized to a pressure slightly less than that of the pilot fuel pressure to inhibit or prevent leakage of gaseous fuel past a fluid seal cavity in the injector.
As disclosed in U.S. Pat. No. 6,298,833 (the '833 patent), issued Oct. 9, 2001, which is also co-owned by the Applicant, it is known to dynamically control sealing-fluid pressure to ensure that gaseous fuel pressure is slightly less than pilot fuel pressure for all engine operating conditions. A pressure-balancing system, which includes a pressure-balancing device such as a dome-loaded regulator, reduces the pressure differential between the sealing-fluid (the pilot fuel) and the gaseous fuel used in the dual fuel injector. At the same time, the pressure balancing system dynamically balances the sealing-fluid pressure such that the gaseous fuel pressure is equal to or slightly less than the pressure of the pilot fuel within the injection valve. The pressure differential between the gaseous fuel and the pilot fuel can be maintained throughout the operating range of engine speeds, engine loads, and fuel cut-off conditions so as to inhibit or prevent leakage of compressible gaseous fuel into the pilot fuel. The reduced pressure differential between the gaseous fuel and the pilot fuel also reduces leakage of pilot fuel into the gaseous fuel.
Existing calibration techniques for natural gas engines employing dual fuel injectors of the type of the '598 patent and diesel as a pilot fuel focus on the calibration of diesel rail pressures required to support emission and fuel usage requirements across the operating range of the engine. One reason for these techniques is integration into the control system of the base diesel engine which operates based on diesel rail pressures. However, in systems that regulate natural gas rail pressure from diesel rail pressure, for example as disclosed in the '833 patent, it has been observed that the pressure differential between the diesel and natural gas rails is not consistent. Unit to unit variations and system aging cause the pressure differential between the pilot fuel and natural gas to change from engine to engine and over time. This causes emission, fuel usage and engine torque targets to vary from what were expected. Since natural gas is the main fuel that determines emissions and fuel usage, when diesel rail pressure is calibrated on a calibration engine, in actual practice the diesel pressure is determined based on the natural gas pressure that meets the emission and fuel usage targets. However, since the pressure differential between the diesel and natural gas rails varies from engine to engine and over time then the natural gas rail pressure will also vary from what it was on the calibration engine. As a result engines tend to operate close to but not at the gas rail pressure within a preferred range of tolerance that meets optimum emission and fuel usage targets.
The present method and apparatus provide an improved technique for controlling fuel pressure in a gaseous fuelled internal combustion engine.